The present invention relates to a steering force control device for obtaining a proper steering force by controlling a hydraulic reaction mechanism in a power steering apparatus in accordance with the various traveling conditions of a vehicle.
In a power steering apparatus for reducing the steering wheel operating force (steering force) of a vehicle, a steering force control device has been proposed, which is designed to perform steering force control in accordance with the various traveling conditions, e.g., traveling velocity and steering angle, of a vehicle by using a hydraulic reaction mechanism.
More specifically, the input and output shafts of the power steering apparatus are relatively pivoted or restricted by using a reaction plunger for selectively restricting the input and output shafts in accordance with the magnitude of a reactive oil pressure so as to perform steering force control in which the reactive oil pressure is minimized to allow a light steering operation during parking or low-velocity travel of the vehicle, and the reactive oil pressure is increased to allow the driver to perform a steering operation with a sense of rigidity during high-speed travel so as to ensure stability in straight travel.
For example, Japanese Patent Laid-Open Nos. 61-105273 and 61-132466 disclose hydraulic reaction mechanisms of this type, in which part of a reactive oil pressure is supplied to a path branching from a main hydraulic path extending from a pump as a hydraulic source to a power cylinder through a flow path switching valve, and is controlled by a spool valve as a hydraulic reaction control valve so as to be introduced into a hydraulic reaction chamber for moving a reaction plunger.
In such a conventional apparatus, the spool valve serving as the hydraulic reaction control valve is generally operated by an electric actuator such as a solenoid coil or a stepping motor which can generate a required operating force by using an output current from a controller on the basis of detection signals from a vehicle velocity sensor, a steering angle sensor, a torque sensor, and the like. With such electronic control, the hydraulic reaction mechanism is properly operated to perform steering force control in accordance with a vehicle velocity or a steering condition.
The conventional steering force control device having the above-described arrangement and using hydraulic reaction commonly uses an oil pump serving as a hydraulic source on the power steering apparatus side operated by the engine, and a reactive oil pressure is introduced from the pump to the hydraulic reaction mechanism through a reactive oil pressure supply path branching from the main hydraulic path of the power steering apparatus. In this arrangement, the power steering apparatus and the operation of the steering force control device based on the hydraulic reaction have a substantially proportional relationship. Assume that the oil pressure from the pump increases during high-speed travel or a steering operation. In this case, this oil pressure is introduced into the spool valve for hydraulic reaction control to be increased, so that the steering force is excessively increased, posing a problem in terms of the steering force characteristics of the power steering apparatus.
In the conventional steering force control device, the spool valve for hydraulic reaction control controls a reactive oil pressure in accordance with a vehicle velocity, a steering angle, and the like. Assume that the held state of the steering wheel shifts to a steered state. In this case, as an assisting oil pressure for obtaining an auxiliary steering force, produced by the power steering apparatus, increases with an increase in load on the power steering apparatus side, the reactive oil pressure also increases without a time delay. As a result, the input torque with respect to the output rapidly increases temporarily. If the subsequent input torque/output characteristics exhibit that an increase in output is large as compared with an increase in input torque in such a state, the input torque difference between these characteristic changing portions makes the driver feel discomfort.
Such changes in characteristics will be described below with reference to FIG. 3. When a held state indicated by "O" in FIG. 3 shifts to a steered state, the input torque rapidly changes as indicated by a portion O-A in FIG. 3. Thereafter, as indicated by a portion A-P in FIG. 3, the increase in output greatly changes with the increase in input torque. The input torque difference between these characteristic changing portions (O-A, A-P) makes the driver feel discomfort.
In such a shift from a held state to a steered state, input torque/output characteristics which the driver feels natural should be smooth characteristics, as indicated by a curve O-B-P in FIG. 3. However, it is difficult for the conventional apparatus to obtain such characteristics.
Especially in the conventional apparatus, such a problem cannot be avoided because the power steering apparatus and the steering force control device based on hydraulic reaction share the hydraulic source, and the reactive oil pressure varies depending on variations in oil pressure in the main hydraulic path on the power steering apparatus side. Therefore, a demand has arisen for measures to eliminate such inconvenience.